OF PLANOCERA ELLIPTICA. 
17 
what in size, and developed into epithelian cells, according to a general physiological 
law. If, perchance, they congregate immediately on being detached from the yolk 
and before they become epithelian, then, still in possession of their own cellular life, 
they, for a time, will constitute a little spheroid, which will acquire vibrillse, in 
accordance with the organic law of development of a large portion of the animal 
kingdom. These little bodies are the so-called Cosmella arcichnoides , described 
by Alex. Nordmann as parasitic animals in the ovum of Tergipes edwardsii , a nu- 
dibranchiate mollusc.* When this spheroid is of a certain size, it sometimes happens 
that a small embryo issues from it, with, a shape altogether similar to the embryos 
formed by the entire mass of the yolk. 
It is my intention to dwell more at length upon these loose cells in another Paper, 
now in preparation, and in which their physiological signification will be thoroughly 
examined. 
X. 
THE EMBRYO. 
§> 1. Between the period of the division of the yolk and that of the first manifesta- 
tion of the embryo, there is a period of apparent repose, which lasts from four to five 
days. The vitelline sphere seems to undergo no process of any kind ; the labor is 
undoubtedly beyond the reach of our investigations, for, that something is going on 
in it, cannot be questioned for a moment: life once started, keeps acting until it ceases 
definitely its actions. During this period the vitelline mass, from opaque (figs. 46 
and 47), acquires two transparent diameters (fig. 48), which are transformed into 
four transparent internal cavities (fig. 49), sometimes a single, but larger central cavi- 
ty is observed (fig. 50), which is less distinct in others (fig. 52 and 53), probably in 
the latter the embryonic substance has not yet acquired all its transparency: a phe- 
nomenon at all events taking place during this period of rest and preceding that of 
motion. 
§ 2. For, now the embryo begins to"move, the most wonderful sight which an ob- 
server may behold. When numbers of them are witnessed at once, some may be seen 
revolving around an ideal axis forty turns a minute, whilst in others the movement is 
scarcely perceptible. And between these two extremes of motion, there are all in- 
termediate degrees of velocity. According to the will of the animal, the velocity may 
be increased or diminished, so that the same individual moves alternatively with dif- 
ferent degrees of velocity. When passing from one degree into another, this does 
not take place according to the mechanical law of uniform acceleration, but the two 
* Essai d’une monographic da Tergipes edwardsii. — Ann. Sc. Nat. 3d Ser. Y. 1846, 109, PI. I. 
